There is a considerable need in both the military and commercial sectors for quiet, efficient and lightweight electric power sources that have improved power density. Military applications include, but are not limited to, submersibles, surface ships, ground vehicles, air vehicles, portable/mobile field generating units, and low power units (i.e., battery replacements). For example, the military has a strong interest in developing high specific energy electrical power sources for unmanned airborne vehicles. Commercial applications include transportation (i.e., automotive, bus, truck and railway), communications, on-site cogeneration and stationary power generation.
Much of this interest has been spurred by environmental problems related to gasoline-powered internal combustion engine and the exhaust gases generated by same. One possible solution to these environmental problems is the use of fuel cells. Fuel cells are highly efficient electrochemical energy conversion devices that directly convert the chemical energy of fuels (such as hydrogen, methanol, and the like) into electrical energy and generate innocuous byproducts such as water.
Significant research and development activity has focused on the development of proton-exchange membrane fuel cells. Proton-exchange membrane fuel cells have a polymer electrolyte membrane disposed between a positive electrode operating as a cathode and a negative electrode operating as an anode. The polymer electrolyte membrane is composed of an ion-exchange polymer (i.e., ionomer). Its role is to provide a means for ionic transport and prevent mixing of the fuel and the oxidant.
Solid polymer electrolyte fuel cells (SPEFCs) are an ideal source of quiet, efficient, and lightweight power. While batteries have reactants contained within their structure which eventually are used up, fuel cells most commonly use air and hydrogen which can be continuously supplied to them, allowing them to continue operation without interruption. Their fuel efficiency is high (45 to 50 percent), they do not produce noise, operate over a wide power range (10 watts to several hundred kilowatts), and are relatively simple to design, manufacture and operate. Further, SPEFCs currently have the highest power density of all fuel cell types. In addition, SPEFCs do not produce any environmentally hazardous emissions such as NOx and SOx (typical combustion by-products).
The traditional SPEFC contains a solid polymer ion-exchange membrane that lies between two gas diffusion electrodes, an anode and a cathode, each commonly containing a metal catalyst supported by an electrically conductive material. The gas diffusion electrodes are exposed to the respective reactant gases, the reductant gas and the oxidant gas. An electrochemical reaction occurs at each of the two junctions (three phase boundaries) where one of the electrodes, electrolyte polymer membrane and reactant gas interface.
During fuel cell operation, hydrogen permeates through the anode and interacts with the metal catalyst, producing electrons and protons. The electrons are conducted via an electrically conductive material through an external circuit to the cathode, while the protons are simultaneously transferred via an ionic route through the polymer electrolyte membrane to the cathode. Oxygen permeates to the catalyst sites of the cathode, where it gains electrons and reacts with protons to form water. Consequently, the products of the SPEFC's reactions are water, electricity and heat. In the SPEFC, current is conducted simultaneously through ionic and electronic routes. Efficiency of the SPEFC is often dependent on its ability to minimize both ionic and electronic resistivity to these currents.
Ion exchange membranes play a vital role in SPEFCs. In SPEFCs, the ion-exchange membrane has two functions: (1) it acts as the electrolyte that provides ionic communication between the anode and cathode; and (2) it serves as a separator for the two reactant gases (e.g., O2 and H2). In other words, the ion-exchange membrane, while serving as a good proton transfer membrane, must also have low permeability for the reactant gases to avoid cross-over phenomena that reduce performance of the fuel cell. This is especially important in fuel cell applications in which the reactant gases are under pressure and the fuel cell is operated at elevated temperatures.
Fuel cell reactants are classified as oxidants and reductants on the basis of their electron acceptor or electron donor characteristics. Oxidants include pure oxygen, oxygen-containing gases (e.g., air) and halogens (e.g., chlorine). Reductants include hydrogen, carbon monoxide, natural gas, methane, ethane, formaldehyde and methanol. Most SPEFCs employ hydrogen as the reductant, oxygen as the oxidant and protons as the current-carrying ion in the solid polymer electrolyte membrane, and consequently water is produced at the cathode as a waste product. In such SPEFCs, optimized proton and water transports of the membrane and proper water management are crucial for efficient fuel cell application. Dehydration of the membrane reduces proton conductivity, and excess water can lead to swelling of the membranes. Inefficient removal of by-product water can cause flooding of the electrodes hindering gas access. Both of these conditions lead to poor cell performance.
Despite their potential for many applications, SPEFCs have not yet been widely commercialized due to unresolved technical problems and high overall cost. Many of these factors impacting the commercialization of the SPEFC are related to deficiencies of today's leading membranes and electrodes. Platinum is routinely used as a catalyst in the electrodes, and it is highly desirable to minimize electrodes catalyst loading in order to reduce cost. However, hydrogen generated by hydrocarbon reforming, which is an inexpensive hydrogen source, is frequently contaminated with trace impurities, such as carbon monoxide, which poison platinum catalysts. Extra processing required to remove these impurities from reformate hydrogen is very costly, as is the addition of extra platinum catalyst to electrodes to compensate for reduced activity in the presence of impurities. It would be desirable to develop a SPEFC capable of operating at temperatures above 120° C., at which temperatures the catalyst poisoning is reduced. An added benefit and cost reduction would be a substantial decrease in the amount of cooling required for the fuel cell. However, today's ion-exchange membranes can not be operated at temperatures much above 90° C. to 95 ° C. due to loss of conductivity and/or insufficient mechanical strength at higher temperatures.
Many current membranes are not suitable for use in low temperature (e.g., less than 100° C.) direct methanol fuel cell applications. Current membranes permit excessive methanol crossover in liquid feed direct methanol fuel cells which decreases operational efficiency of the fuel cell (losses are dependent on actual operating conditions, but is often equivalent to a current density loss of about 50 to 200 mA/cm2 @ 0.5V). This crossover results in poor fuel efficiency as well as limited performance levels.
To make the SPEFC commercially viable (especially in automotive applications), the membranes employed must operate at elevated/high temperatures (>120° C.) so as to provide increased power density, and limit catalyst sensitivity to fuel impurities. This would also allow for applications such as on-site cogeneration (high quality waste heat in addition to electrical power).
Several polymer electrolyte membranes have been developed over the years for application as solid polymer electrolytes in fuel cells. However, these membranes have significant limitations when applied to liquid-feed direct methanol fuel cells and to hydrogen fuel cells. The membranes in today's most advanced SPEFCs do not possess the required combination of ionic conductivity, mechanical strength, dehydration resistance, chemical stability and fuel impermeability (e.g., methanol crossover) to operate at elevated temperatures.
DuPont developed a series of perfluorinated sulfonic acid membranes known as Nafion® membranes. The Nafion® membrane technology is well known in the art and is described in U.S. Pat. Nos. 3,282,875 and 4,330,654. Unreinforced Nafion® membranes are used almost exclusively as the ion exchange membrane in present SPEFC applications. This membrane is fabricated from a copolymer of tetrafluoroethylene (TFE) and a perfluorovinyl ethersulfonyl fluoride. The vinyl ether comonomer is copolymerized with TFE to form a melt-processable polymer. Once in the desired shape, the sulfonyl fluoride group is hydrolyzed into the ionic sulfonate form.
The fluorocarbon component and the ionic groups are incompatible or immiscible (the former is hydrophobic, the latter is hydrophilic). This causes a phase separation, which leads to the formation of interconnected hydrated ionic “clusters”. The properties of these clusters determine the electrochemical characteristics of the polymer, since protons are conducted through the membrane as they “hop” from one ionic cluster to another. To ensure proton flow, each ionic group needs a minimum amount of water to surround it and form a cluster. If the ionic group concentration is too low (or hydration is insufficient) proton transfer will not occur. At higher ionic group concentrations (or increased hydration levels) proton conductivity is improved, but membrane mechanical characteristics are sacrificed.
As the membrane temperature is increased, the hydrated membrane softens and becomes susceptible to mechanical failure by developing a pinhole or allowing a contact short between the electrodes. At even higher temperatures (230° C.), the fluorocarbon phase melts and permits the ionic domains to “dissolve” (phase inversion of Nafion®).
There are several mechanisms that limit the performance of Nafion® membranes in fuel cell environments at temperatures above 100° C. In fact, these phenomena may begin at temperatures above even 80° C. Mechanisms include membrane dehydration, reduction of ionic conductivity, loss of mechanical strength via softening, and increased parasitic losses through high fuel permeation.
Crossover problems with Nafion® membranes are especially troublesome in liquid feed direct methanol fuel cell applications, where excessive methanol transport (which reduces efficiency and power density) occurs. Methanol-crossover not only lowers fuel utilization efficiency but also adversely affects the oxygen cathode performance, significantly lowering cell performance.
The Nafion® membrane/electrode is also very expensive to produce, and as a result it is commercially prohibitive for many applications. Reducing membrane cost is crucial to the commercialization of SPEFCs. It is estimated that membrane cost must be reduced by at least an order of magnitude from the Nafion® model for SPEFCs to become commercially attractive.
Another type of ion-conducting membrane, Gore-Select® (commercially available from W. L. Gore), is currently being developed for fuel cell applications. Gore-Select® membranes are further detailed in a series of U.S. Patents (U.S. Pat. Nos. 5,635,041, 5,547,551 and 5,599,614).
Gore discloses a composite membrane consisting of a porous Teflon® film filled with a Nafion® or Nafion®-like ion-conducting solution. Although it has been reported to show high ionic conductance and greater dimensional stability than Nafion® membranes, the Teflon® and Nafion® materials selected and employed by Gore as the film substrate and the ion-exchange material, respectively, may not be appropriate for operation in high temperature SPEFCs. Teflon® undergoes extensive creep at temperatures above 80° C., and Nafion® and similar ionomers swell and soften above the same temperature. This can result in the widening of interconnected channels in the membrane and allow performance degradation, especially at elevated temperatures and pressures.
Further, Gore-Select®, as well as many other types of perfluorinated ion-conducting membranes (e.g., Aciplex from Asahi Chemical, Flemion® from Asahi Glass, Japan), are just as costly as Nafion®, since these membranes employ a high percentage of perfluorinated ionomers.
In an effort to reduce costs and move toward potential commercialization of SPEFCs, ion-exchange membranes that are less expensive to produce also have been investigated for use in polymer electrolyte membrane fuel cells.
Poly(trifluorostyrene) copolymers have been studied as membranes for use in polymer electrolyte membrane fuel cells. See e.g., U.S. Pat. No. 5,422,411. However, these membranes are suspected to have poor mechanical and film forming properties. In addition, these membranes may be expensive due to the inherent difficulties in processing fluorinated polymers.
Sulfonated poly(aryl ether ketones) developed by Hoechst AG are described in European Patent No. 574,891,A2. These polymers can be crosslinked by primary and secondary amines. However, when used as membranes and tested in polymer electrolyte membrane fuel cells, only modest cell performance is observed.
Sulfonated polyaromatic based systems, such as those described in U.S. Pat. Nos. 3,528,858 and 3,226,361, also have been investigated as membrane materials for SPEFCs. However, these materials suffer from poor chemical resistance and mechanical properties that limit their use in SPEFC applications.
Solid polymer membranes comprising a sulfonated poly(2,6 dimethyl 1,4 phenylene oxide) alone or blended with poly(vinylidene fluoride) also have been investigated. These membranes are disclosed in WO 97/24777. However, these membranes are known to be especially vulnerable to degradation from peroxide radicals.
The inherent problems and limitations of using solid polymer electrolyte membranes in electrochemical applications, such as fuel cells, at elevated/high temperatures (>100° C.) have not been solved by the polymer electrolyte membranes known in the art. Specifically, maintaining high ion conductivity and high mechanical strength, resisting dehydration and other forms of degradation remain problematic, especially at elevated operating temperatures. As a result, commercialization of SPEFCs has not been realized.
It would be highly desirable to develop an improved solid polymer electrolyte membrane with high resistance to dehydration, high mechanical strength and stability to temperatures of at least about 100° C., more preferably to at least about 120° C.
It would also be highly desirable to develop a membrane with the afore-mentioned characteristics that would be suitable for use in a hydrogen or methanol fuel cell and that would provide an economical option to currently available membranes. The development of such a membrane would promote the use of SPEFCs in a variety of highly diverse military and commercial applications, and would be beneficial to industry and to the environment.